This invention relates to metal oxide varistor configurations, and more particularly to configurations useful in the protection of polyphase electrical circuits.
Polyphase electrical power distribution systems or electrical devices, such as motors, often require surge voltage protection devices to guard against transient conditions to which these devices are susceptible. Metal oxide varistors, particularly those comprising a major portion of zinc oxide along with other metal oxide additives such as bismuth oxide, cobalt oxide, chromium oxide, and other metal oxide additives, operate in a nonlinear fashion so as to limit the voltage appearing across the terminals of the varistor. These metal oxide varistors are capable of diverting destructive energy surges from a device to which they are electrically connected so that the energy surge is safely absorbed by the varistor. The need and utility of such varistor protection is further detailed in U.S. Pat. No. 3,894,274 issued July 8, 1975 to G. M. Rosenberry, Jr. and assigned to the same assignee as the present application. This patent particularly describes the utility of metal oxide varistor protection for relatively low voltage, polyphase, alternating current motors.
Varistors for protection of polyphase circuits are conveniently packaged in a single device. Prior devices incorporating metal oxide varistors for polyphase voltage protection have typically configured the varistors on a flat disk, one side of which is coated with an electrical contact, the other face of the disk being coated with four half-semicircular electrode contacts. Further, the spacing between the four adjacent contacts is increased by providing grooves in the varistor material between said contacts so as to insure electrical conduction between the two varistor faces rather than between adjacent electrodes on a single face of the varistor. This configuration has a relatively high diameter-to-thickness ratio, particularly in low voltage applications in which the thickness of the varistor disk must be selected to be relatively small so as to provide a low breakdown, limiting, or so-called clamping voltage for the varistor. This high diameter-to-thickness ratio yields a varistor which is relatively easily broken, particularly if grooves are present between adjacent electrode contacts. Moreover, such a varistor configuration does not exhibit a high degree of flexiblity with respect to the placement of electrode leads for external circuit connection.
U.S. Pat. No. 3,693,053 issued Sept. 19, 1972 to T. E. Anderson and assigned to the same assignee as the present invention discloses a metal oxide varistor configuration particularly suitable for polyphase transient voltage suppression. However, the configuration employed in the aforementioned Anderson patent requires an elaborately shaped varistor body which is not easily manufactured and which exhibits low mechanical strength, particularly in low voltage configurations.